The present invention relates to a multi-hop network, a node, a multicast route control method, and a program used for multicast communications respectively.
There are some unicast route control methods used in general radio multi-hop networks. For example, a non-patent document 1 (T. Clausen et al, “Optimized Link State Routing Protocol for Ad Hoc Networks”, IEEE INMIC, December, 2001) discloses such a radio multi-hop control method. Hereunder, there will be described OLSR (Optimized Link State Routing) disclosed in the non-patent document 1.
In case of the OLSR, a communication device broadcasts Hello messages to its neighbor nodes at predetermined sending intervals after the system is started up. Those Hello messages are not forwarded. The messages are received only by the communication devices (hereinafter, to be referred to as nodes) existing within a radio transmission range.
If a node receives such a Hello message from another node, the node holds the message information in an neighbor node table while the message is valid. If the node sends a Hello message to another node, the node includes a list of the IP addresses of all the nodes recorded such way in the neighbor node table into the Hello message. Receiving such a Hello message that includes the neighbor node information, the receiver node selects an MPR (Multi Point Reply) node and sends a TC (Topology Control) message to the MPR node.
An MPR node means a forwarding node that forwards route control messages (TC messages or like) to be received by all nodes in the subject network. The MPR node selection is made by calculating a set of neighbor nodes that cover all the nodes existing two hops away from the calculating node. An MPR node selected by such a node is notified to its neighbor nodes through the Hello message. Consequently, the neighbor nodes can know they are required respectively to forward the message if receiving such a control message as a TC packet or the like and it is required to be notified to every node in the network from the node selected as an MPR node by itself.
The TC message notifies the link information of a source node (usually, the information of a link with the selected MPR node) to every node in the subject network. The TC message is created at a preset sending interval and forwarded by the MPR node to be notified to every node in the network. The link information set in such a TC message received from another node is stored in a topology table. Each communication node creates a network topology graph from the link information recorded in the topology table to calculate the shortest route to each destination communication node. And according to the result of this calculation, the object communication forwarding route is set.
A non-patent document 2 (Anis Laouiti et al, “Multicast Optimized Link State Routing”, Inria Research Report No. 4721, February, 2003, (ftp://ftp.inria.fr/INRIA/publication/publi-pdf/RR/RR-4721.pdf)) discloses a multicast control method (hereunder, to be referred to as the M-OLSR) employed for radio multi-hop networks. The method is expanded from the OLSR mechanism disclosed in the non-patent document 1.
In case of the M-OLSR, each multicast packet sender node floods the SOURCE_CLAIM message all over the subject network. Receiving such a SOURCE_CLAIM message, the receiver node that is to receive the multicast sends a CONFIRM_PARENT message to the next hop node (a node required to forward packets addressed to the sender node) so as to be forwarded to the sender node. The relay node that has received the CONFIRM_PARENT message thus forwards the CONFIRM_PARENT message to the next hop node set by the sender node similarly unless the relay node is neither the receiver node nor relay node of the multicast, thereby setting a multicast forwarding table. If the relaying node is already set as the receiver or relay node of the multicast communication, the node sets a multicast table and does not forward the CONFIRM_PARENT message. The sender node keep sending the SOURCE_CLAIM message and the receiver and forwarding nodes keep sending the CONFIRM_PARENT message periodically to maintain the multicast forwarding table respectively. If not receiving any messages for a certain period, it is regarded that the sender or receiver node has disappeared, so that the subject entry in the multicast forwarding table is deleted.
A non-patent document 3 (Carlos de Morais et al, “Multicast over Wireless Mobile Ad Hoc Networks: Present and Future Directions”, IEEE Network Magazine, January/February, 2003) also discloses still another multicast control method employed for radio multi-hop networks. In addition to the above control methods, still other various similar methods have been disclosed. According to the non-patent document 3, such multicast control methods are roughly classified into the tree type method and the mesh type method. In case of the tree type method, a representative node is determined first, then both the sender and the receiver (members) of the multicast communication join in the communications with the representative node, thereby constructing a multicast distribution tree. In case of the mesh type method, a route is formed between each pair (of a sender node and a receiver node). The above-described M-OLSR is also included in this mesh type method.
In case of the conventional mesh type multicast route control method described above, however, because a route is formed between each pair of a sender node and a receiver node, the number of routes to be formed, as well as the number of required control messages increase respectively. Consequently, it is difficult to employ the method for narrow band networks. Furthermore, when the number of routes increases such way and accordingly the number of nodes that forward multicast packets increases, packet sending operations among those nodes come to interfere with each another, thereby causing degradation of communications (confliction of packets, packet losses, increases of delays, etc.).
And in order to avoid such conventional problems, there have been considered a technique that adjusts the frequency of control packets exchanged with other nodes with use of moving information including the self-node moving speed (e.g., refer to a patent document 1 (Japanese Patent No. 3893620).